The present invention relates to an image forming apparatus which develops a static charge image and a magnetic latent image in electrophotography, electrostatic printing, magnetic recording and others, and more particularly to an image forming apparatus using a heat fusing scheme such as heat roller fusing.
A cleanerless process which does not have a cleaner such as a blade on a photoreceptor surface is a technique which is advantageous to a reduction in size of an apparatus or conservation of a toner, and various inventions of the cleanerless process have been disclosed. For example, U.S. Pat. No. 4,727,395 discloses a development/simultaneous cleaning technique in an reversing phenomenon. In particular, this technique is also effective in realization of full color in recent image forming apparatuses and has begun to be adopted in a four-drum tandem mode, and, e.g., Japanese Patent No. 3342217, Jpn. Pat. Appln. KOKAI Publication No. 7-64366 and others disclose examples concerning the cleanerless process having this tandem configuration.
As merits of the cleanerless process, there are simplification of a configuration since a photoreceptor cleaner is not necessary, realization of long duration of life of a photoreceptor because the photoreceptor is not scraped by a cleaner, an improvement in toner consumption efficiency since a post-transfer residual toner is recovered by a developing device for recycling, and others.
On the other hand, as demerits of the same, filming is apt to occur on a photoreceptor surface due to the toner since a blade is not provided on the photoreceptor. Occurrence of filming obstructs a demand for a reduction in fusing temperature in recent years. Further, since a post-transfer residual toner passes through a charged part and an exposed part, there is a problem that a memory is apt to be generated in an image due to an exposure in particular. When the memory is generated, an image quality is deteriorated. Furthermore, in case of a tandem type color image forming apparatus, reverse transfer from a color station on a preceding stage to a color station on a subsequent stage occurs and a mixed color is generated depending on an image to be printed, thereby resulting in a change in hue.
Therefore, prevention of occurrence of a memory and a mixed color has been extensively tried in a color image forming apparatus in particular by forming the toner into a spherical shape and increasing a transfer performance. On the other hand, one of factors of filming is a wax component contained in the toner.
As a method of realizing a spherical shape of the toner, there can be considered a method which forms a spherical shape while heating a polymerized toner manufactured by using a polymerization method or a toner created by a grinding method. In case of the toner manufactured by the polymerization method, there can be considered that a countermeasure against filming is taken by providing a polymeric resin layer on an outermost layer of the toner whilst transfer properties are improved. On the other hand, in case of the toner formed into a spherical shape while heating the toner created by the grinding method, the wax is apt to elute on the toner surface and a filming performance is deteriorated. When using such a toner, taking a countermeasure against filming and improving the transfer properties can be considered by setting a pressure of a transfer nip section where filming is generated, a nip width and others as predetermined conditions.
Prevention of filming of the toner by improving surface properties of a photoreceptor can be considered. Avoidance of filming of the toner on a photoreceptor by application of a mold releasing agent such as a metal soap on the photoreceptor has been also tried.